1. Field of the Invention
The present invention generally relates to a spindle motor and more particularly relates to a spindle motor for rotating a disk-shaped storage medium, such as magnetic disks, typified by hard disks; various types of optical disks; and magnetic optical disks.
2. Description of the Related Art
Recently, hard disk drive devices are being installed in mobile devices. These mobile devices are battery-powered, and in order to ensure longer battery operating time, the spindle motors (hereinafter referred to as motors) used in the hard disk drive devices are required to have severely low power consumption.
FIGS. 10 and 11 are sectional views of conventional motors. In the conventional motors shown in FIG. 10 and 11, the circular rotor magnet 2 is fixed to a lower side and in a radially-outward portion of the rotor hub 1. In addition, the rotor magnet 2 opposes the stator 3 with a gap maintained between the rotor magnet 2 and the stator 3 in the radially-outward direction of the rotor magnet 2.
The rotor hub 1 of the conventional motor shown in FIG. 10 includes a circular magnetic shield portion 1a made of a ferromagnetic material. The magnetic shield portion 1a is a circular surface provided on the bottom (rotor magnet side) surface of the rotor hub 1, when viewed from the rotor magnet 2 side, and covers the upper surface (the surface on a side near the magnetic disk) of the rotor magnet 2. This arrangement prevents the magnetic flux generated by the rotor magnet 2 from damaging the data stored on the magnetic disks and allows magnetic flux to flow smoothly from the rotor magnet 2 to the stator 3.
In the conventional motor shown in FIG. 10, the upper surface of the rotor magnet 2 abuts the bottom surface of the magnetic shield portion 1a, and the magnetic field short-circuits at the abutted portion, forming another magnetic circuit therebetween. Therefore, the magnetic flux, which flows into stator 3, decreases. As a result, the conventional motor generates less torque, requires more magnetizing current, and increases the power consumption.
In order to prevent the short-circuiting explained above, a gap may be provided between the upper surface of the rotor magnet 2 and the bottom surface of the magnetic shield portion 1b of the conventional motor shown in FIG. 11. However, providing this gap makes it difficult to accurately axially position the rotor magnet 2. Accurate axial positioning of the rotor magnet 2 has a substantial influence on the performance of motors used in mobile devices, particularly on the performance of motors whose axial direction thickness is 10 mm or less.